1. Field of the Invention
The present invention relates to an objective lens system which is to be used in combination with optical systems for microscopes and so on.
2. Description of the Prior Art
The conventional objective lens systems, in particular, those which have high magnifications and large numerical apertures needed to comprise large numbers of cemented lens components and use anomalous dispersion glass materials for favorably correcting aberrations, in particular, chromatic aberration. Accordingly, the objective lens systems were inevitably expensive, and it was impossible to design objective lens systems for the ultraviolet region and the infrared region which require lens elements made of glass materials selectable within a restricted range.
In recent days, attention is paid to optical systems which use diffractive optical elements as lens elements for composing the lens systems. As conventional examples of objective lens systems which use diffractive optical elements and are of a type which is similar to that of the objective lens system according to the present invention, there are known those disclosed by Japanese Patents Kokai Publication No. Sho 63-77,003, Kokai Publication No. Sho 63-155,432, Kokai Publication No. Sho 59-33,636, Kokai Publication No. Sho 60-247,611, Kokai Publication No. Hei 2-1,109 and Kokai Publication No. Hei 4-361,201.
Diffractive optical elements which utilize the diffraction phenomenon of light are described in "NEW METHODS OF DESIGNING HOLOGRAPHIC OPTICAL ELEMENTS" by William C. Sweatt (SPIE. VOL. 126, P46-53, 1977), etc. The operating principle of the diffractive optical elements can be summarized as follows:
An ordinary glass material refracts a ray as illustrated in FIG. 28 or according to Snell's law which is expressed by the following formula (1): EQU n sin .theta.=n' sin .theta.' (1)
wherein the reference symbol n represents a refractive index of a medium located on the side of incidence of the glass material, the reference symbol n' designates a refractive index of a medium located on the side of emergence of the glass material, the reference symbol .theta. denotes an angle of incidence of a ray and the reference symbol .theta.' represents an angle of emergence of the ray.
Speaking of the diffraction phenomenon, on the other hand, a ray is diffracted as illustrated in FIG. 29 or according to the law of diffraction which is expressed by the following formula (2): EQU n sin .theta.-n'sin .theta.'=m.lambda./d (2)
wherein the reference symbol m represents an order of a diffracted ray, the reference symbol .lambda. designates a wavelength and the reference symbol d denotes a spacing between two adjacent grooves formed on a diffractive optical element of interest.
Means by "diffractive optical element" is an optical element which is configured so as to deflect rays according to the law expressed by the above-mentioned formula (2). Though shielding sections and transmitting sections are disposed regularly at the spacing d in FIG. 29, it is possible to obtain a high diffraction efficiency by forming a diffractive surface having a saw-tooth-like sectional shape on a transparent material or approximating the sectional shape by the binary technique.
Now, description will be made below of advantages obtainable by using such a diffractive optical element:
The following formula (3) applies to a thin refractive lens element: EQU 1/f=(n-1)(1/r.sub.1 -1/r.sub.2) (3)
wherein the reference symbol f represents a focal length of the refractive lens element, the reference symbols r.sub.1 and r.sub.2 designate radii of curvature on a surface of incidence and a surface of emergence respectively of the refractive lens element, and the reference symbol n denotes a refractive index of the refractive lens element.
Differentiation of both the sides of the above-mentioned formula (3) by wavelength .lambda. gives the following formula (4): EQU df/d.lambda.=-f(dn/d.lambda.)/(n-1) (4) EQU .thrfore..DELTA.f=-f{.DELTA.n/(n-1)}
Since .DELTA.n/(n-1) which is obtained by removing the effect produced by the multiplication of the coefficient represents a dispersion characteristic of the lens element, a dispersion value .nu. of the lens element can be defined as follows: EQU .nu..ident.(n-1)/.DELTA.n (5)
Hence, the lens element has a dispersion value (Abbe's number) in the visible region which is expressed as follows: EQU .nu..sub.d =(n.sub.d -1)/(n.sub.F -n.sub.C) (6)
On the other hand, the diffractive optical element has a focal length expressed by the following formula (7): EQU f=h/(n' sin .theta.')=(d.sub.h h)/(m.lambda.) (7)
wherein the reference symbol f represents a focal length of the diffractive optical element and the reference symbol d.sub.h designates a groove spacing at a height h of incident parallel rays.
Since d.sub.h h is constant for an aplanatic diffractive optical element when it is considered according to the paraxial theory, f can be expressed as f=con./.lambda. ("con" is an abbreviation of a constant). Differentiation of both the sides of f=con./.lambda. by .lambda. gives the following formula (8): EQU df/d.lambda.=-con./.lambda..sup.2 =-f/.lambda. (8) EQU .DELTA.f=-f(.DELTA..lambda./.lambda.)
Since .DELTA.n/(n-1) is equal to .nu., we obtain .nu.=.lambda./.DELTA..lambda. from the formulae (4) and (8). Therefore, the diffractive optical element has an Abbe's number expressed by the following formula (9) in the visible region: EQU .lambda..sub.d =.lambda..sub.d /(.lambda..sub.F -.lambda..sub.C)=-3.453 (9)
Hence, the diffractive optical element has a negative dispersion value which is very large in absolute. Since the ordinary glass materials have dispersion values of approximately 20 to 95, it will be understood that the diffractive optical element has a remarkably inverse dispersion characteristic. Similar calculations lead to an understanding that the diffractive optical element has anomalous dispersion characteristic.
Out of the objective lens systems mentioned as the conventional examples, the objective lens systems disclosed by Japanese Patents Kokai Publication No. Sho 63-77,093, Kokai Publication No. Sho 63-155,432, Kokai Publication No. Sho 59-33,636 and Kokai Publication No. Sho 60-247,611 are pickup lens systems for optical discs. Each of these objective lens system comprises one or two diffractive optical elements or a single refractive optical element (lens element) and a single diffractive optical element, fundamentally uses a monochromatic light source and does not utilize the function of the diffractive optical element for correcting chromatic aberration.
Further, the objective lens systems disclosed by Japanese Patents Kokai Publication No. Hei 2-1,109 and Kokai Publication No. Hei 4-361,201 are steppers to be used with photographic lens systems and composed of lens elements made only of fused silica without using cemented lens components. The former objective lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-1,109, in particular, is characterized in that it comprises a diffractive optical element disposed at a location of a pupil of the objective lens system, whereas the latter objective lens system disclosed by Japanese Patent Kokai Publication No. Hei 4-361,201 is characterized in that it utilizes rays diffracted by a marginal portion of the diffractive optical element rather than those diffracted by a central portion thereof.
However, the conventional pickup lens systems are not usable as objective lens systems for microscopes which must have more complicated compositions. The conventional stepper type lens systems may be usable as objective lens systems for microscopes having low magnifications, but are unusable as objective lens systems for microscopes which have high magnifications and large numerical apertures, since it is necessary for correcting chromatic aberration in an objective lens systems only with the diffractive optical elements that they have strong refractive powers and groove pitches which are too narrow for practical manufacturing.